The long-term goals of this research are to understand the adaptive and compensatory capabilities of the gaze stability systems following vestibular lesions and to identify the mechanisms that drive the adaptation of the vestibulo-ocular reflexes. An understanding of the adaptive mechanisms will be pertinent to motor learning, a fundamental question in neuroscience, and will have practical implications on our understanding of how humans adapt to vestibular disorders. A functional, long-term outcome of this research will be the development of more appropriate rehabilitation strategies for patients with vestibular disorders. These rehabilitation strategies may serve as models for the rehabilitation of other neuromuscular movement disorders. The first aim of the study is to determine if the irregularly discharging vestibular afferents are responsible for the generation of the nonlinear component of the angular vestibulo-ocular reflex (AVOR). This will be determined in a series of behavioral and electrophysiologic experiments in squirrel monkeys. Monkeys will undergo spectacle-induced adaptation of the AVOR; reversible inactivation of the irregular afferents will be induced at selected time epochs during the adaptation stimulus. Pre- and post-adaptation measures of the AVOR will be used to determine the role of the irregular afferent signal in the generation of the AVOR. In the same animals, the discharge properties of both regular and irregular afferents will be recorded using identical rotational and electrical stimuli. The second aim is to determine if optically induced adaptation of the AVOR will reduce the deficit and asymmetry between ipsilesional and contralesional rotations following a unilateral vestibular deficit (UVD). Following recovery from a UVD, monkeys will be adapted with either magnifying spectacles or with CSA using direction of rotation as the context cue, and the recovery from this induced increase in AVOR gain will be monitored to determine if adaptation will lead to reduced functional deficits.